The electron density of the ionosphere varies as a function of geographic location and time. The International Civil Aviation Organization (ICAO) is developing standards to address the threat to navigation and guidance systems in aircraft stemming from an ionosphere anomaly, i.e., extreme changes in electron density of the ionosphere. The standard development has shown it is critical that ground monitoring for delay gradients in satellite signals due to an ionosphere anomaly is absolute and not relative to previous satellite measurements. It is important to detect an extreme gradient from the very first set of measurements taken from a satellite as it moves into view of a ground-based GPS augmentation system.
The GBAS (Ground Based Augmentation System) Approach Service Type D (GAST D) standard shares the mitigation of ionosphere gradient errors between the ground and airborne subsystems. This leads to the need for an ionosphere gradient monitor (IGM) located in the ground subsystem in order to ensure the integrity of the system is not compromised in the presence of an ionosphere gradient. If an ionosphere gradient is present, then there will be a measureable phase difference between a reference receiver pair for a particular satellite after the satellite has been differenced with the average of all the other satellites in order to remove common mode errors. Due to non-common satellite errors, this differencing with the average of all the other satellites limits the IGM to carrier phase differences to no greater than ±λ/2. Thus, to avoid carrier phase differences greater than ±λ/2, the baseline separation of reference receivers is limited based on the wavelength of the satellite signal and largest ionosphere gradient required to be detected.